Bright copper plating process



CONCENTRATION OF C112 SE MG/L.)

P 1958 an. BOEL'II'ER, JR 2,854,389

BRIGHT COPPER PLATING PROCESS Filed April 30, 1956 SOLUBILITY OF Cuz sE IN COPPER CYANIDE PLATING BATHS so c Cu CN KOH 7.3

FREE KCN'(oz./cAL.)

]NVENTOR.- EDWIN D. BOELTER JR.

. ATTORNEY Cu CN Cu CN CU CN 1200 v 4.7(0Z/GAL.) 9.6 (oz/GAL.) l4.5 (oz/GAL) IOOO ' cyanide electroplating baths.

BRIGHT COPPER PLATING PROCESS Edwin D. Boelter, Jr., Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application April 30, 1956, Serial No. 581,762

8 Claims. (Cl. 20452) This application relates to the electrodeposition of bright copper from an aqueous copper cyanide plating bath. In one aspect it is concerned with addition agents to be employed in copper plating. In another aspect it is concerned with a process utilizing these agents.

It is in part a continuation of my copending application Serial Number 426,950, filed May 3, 1954. It is also related to my copending application Serial Number 543,496, filed October 28, 1955.

GENERAL CONSIDERATIONS The above-mentioned copending applications disclose that selenides are extremely valuable agents for increasing the plating speed obtainable from aqueous copper Thus current densities of 75-100 a./s. f. (amperes per square foot), or of even much higher values, can readily be. used when a small amount of a selenide is added to the bath. Independent sources, as for example U. S. Patent 2,694,677, show that some selenides, organic selenides in particular, are

also eilective in brightening copper deposited from baths containing them.

One of the few selenides that can be easily prepared in stable form is cuprous selenide. Most of the other inorganic selenides, especially those of the alkali and alkaline earth metals, are hygroscopic and unstable, sometimes igniting When exposed to moist air. Organic selenides are rather difiicult to synthesize in a pure condition and are also somewhat unstable, although less so than alkali metal selenides. Organic selenides do, however, tend to decompose in cyanide baths with consequent loss of the properties valuable in electroplating.

The stability of the cuprous selenide and the fact that it carries no element other than selenium foreign to copper plating baths recommends it for utility therein. It suifers, however, from two disadvantages which have heretofore prevented its commercial acceptance. The first is that it does not dissolve in water and dissolves so slowly in cyanide baths that, before dissolution, it may contaminate the workpiece being plated. The second is that it is only a mediocre brightener and, by itself, cannot produce electroplate which meets required commercial standards.

There are, consequently, two principal objects of this invention. The first is to provide a means for the rapid dissolution of cuprous selenide in an aqueous copper cyanide plating bath. The second is to provide a method for utilizing cuprous selenide as a brightening agentin cyanide-copper plating.

The above-mentioned and yet other objects of the invention are broadly achieved by a process in which .(1) a mixture of cuprous selenide and an excess of a metallic cyanide such as that of potassium or sodium, preferably in solution, is added to a substantially conventional aqueous copper cyanide bath containing a secondary brightener and (2) electroplating is carried out in substantially the normal manner, except that extremely high current densities or plating speeds are optionally utilized.

The figure displays, curves for the solubility of cuprous selenide in solutions of various concentrations of cuprous cyanide useful in carrying out the first step of the broad process.

DISSOLVING THE CUPROUS SELENIDE The addition of cuprous selenide and a metallic cyanide to the bath is preferably accomplished by preliminarily dissolving the cuprous selenide in a very concentrated cyanide solution and adding this solution to the bath. When undissolved cuprous selenide is added to a conventional bath, it remains as a black solid. This solid will eventually dissolve if its quantity does not exceed the solubility of the selenide but the dissolution may re quire many hours, even at the -90 C. temperature customarily employed. Since any solid in the baths may roughen the workpieces, such a slow rate of dissolution is intolerable.

The dissolution of cuprous selenide can be greatly expedited by accomplishing the same in a concentrated solution of a cyanide, preferably potassium cyanide, rather than in water. The curves of the figure explain this effect. They are the results of experiments carried out by adding incremental amounts of Cu Se to conventional plating solutions at C. and then adding KCN until the selenide dissolved. They show that increased free-cyanide greatly increases the solubility of cuprous selenide while increased copper cyanide reduces it. The vertical branches of the curve originate at a point corresponding to a solution concentration indicating the formation of an addition compound represented by the empirical formula K Cu (CN) If the relative. concentration of KCN exceeds that required by this formula the selenide is very soluble. At a lower concentration of KCN, it is much less soluble, An equation for the dis.- solution of the selenide may be written as:

The increased solubility of the cuprous. selenide is accompanied by an increased rate of solution. Thus when cuprous selenide is put into a concentrated solution of potassium or sodium cyanide, it will dissolve almost instantaneously so long as the copper-cyanide ratio does not exceed that of the addition compound mentioned. The rate, of course, depends to some extent on the temperature. The preferred temperature is that of the plat ing bath, i. e. 7590 C., but room temperature (ca. 20 C.) can be used Without greatly lowering the rate of solution. Once dissolved, the cuprous selenide can be placed in the relatively dilute electroplating bath without precipitation.

If it is so desired, the composite KCN.CU Se solutions can be made up literally as described above, that is, by dissolving the selenide in an already prepared cyanide solution. Since the composite solutions are unstable and readily oxidized on exposure to air, they must be used shortly after they are made.

A preferred method of preparing the composite solutions, is by dissolving solid mixtures of comminuted cu.

prous selenide and potassium or sodium cyanide in the, minimum quantity of water needed for dissolving the cyanide. Solutions of 20-40% concentration can theoretically be prepared in this manner. The solids generally dissolved at 7590 C. within a few minutes, particularly if agitation is employed. The dry mixtures can be kept indefinitely without spoilage.

In a procedure alternative to the preliminary dissolution of the Cu Se-KCN mixtures, and less preferred, the solid mixtures are. added directly to the bath. Mixtures containing up to 5-10% Cu Se dissolve fairly readily. To insure rapid and uniform distribution without the possibility of ruining the plate being produced, the prelimiweight when potassium cyanide is employed and about 37.5% by weight when sodium cyanide is the second constituent. Such mixtures will theoretically dissolve completely if suflicient water is present to dissolve the cyanide. As will be evident from one of the examples presented further on, such percentages of cuprous selenide are not, in fact, readily dissolved. A higher percentage of cyanide than that set by the formula given is necessary for ready solubility. In practice, the solid mixture should not contain more than about by weight of Cu -Se with KCN or 2530% with NaCN. The upper branches of the curves of the figure are not vertical to infinity.

A lower limit in the percentage of cuprous selenide in the solid mixtures is set by the requirements of the electroplating bath. At least 1% by weight shouldbe present if the mixture is to have any practical utility. Commercial operation of copper plating baths shows a consumption of aboutl lb. of KCN for each 5 lbs. of copper plated, the cyanide being largely converted to carbonate. Theconsumption of Cu Se under these conditons is such that a 1.1% Cu Se98.9% KCN mixture supplies all the KCN and O1 Se required to operate the bath continuously. If NaCN is used the mixture should contain 1.5% Cu Se. Convenient usable solid mixtures have compositions in the ranges l25% by weight of Cu Se with 99-75% of KCN and 15-30% of Cu Se with 97.5-70% of NaCN.

Solid cuprous selenide can be employed with mixtures of sodium and potassium cyanides as well as with either alone. Solid barium cyanide can also be used with the selenide in stoichiometric substitution for KCN or NaCN and aids in controlling carbonate in the electroplating bath. If Ba(CN) is used alone with the selenide, the concentration of Cu Se in the mixture should not rise above about 17% by weight. Ca(CN) may also be employed but is too hygroscopic for convenience. Still other metallic cyanides, particularly those of the other alkali and alkaline earth metals, may be included in the mixtures.

Any readily soluble material harmless to the bath may actually be admixed with cuprous selenide and soluble cyanide. The primary requisite of the mixtures remains, of course, that they include enough cyanide to insure the speedy dissolution of the cuprous selenide. Cuprous cyanide may be added to the mixtures but decreases the solubility of the selenide and ease of control of dissolution. A complex product of cuprous cyanide and another cyanide, e. g. KCN, is also usable. Additional brighteners can be included but they are not required. Inert diluents such as potassium carbonate can be added but serve no useful purpose. 7

Theforegoing discussion shows that the aspect of the invention concerned with addition agents is not limited to pure mixtures of solid cuprous selenides and solid cyanides. It comprises, in fact, all suitable cuprous selenidealkali or alkaline earth metal cyanide compositions in which the weight of selenide does not exceed. the theoretical weight for ready solubility given above i. e. about 31.2% when KCN is the second constituent and about 37.5% when NaCN is the second. Since theweight ratios vary with the compound utilized, the proportions of material are more conveniently expressed in invariable equivalent weight ratios. Here the term equivalent weight is utilized with a customary meaning and represents the molecular weight divided by the valence. The

theoretical equivalent rate ratio for Cu Se:MCN mixtures limited by the formula of the addition compound is about 0.007 to 0.286. In practice this ratio does not exceed about 0.21. The same ratios are employed where alkaline earth cyanides are substituted for those of the alkali metals. It will be understood that the mixtures, to be suitable for copper plating, should be substantially free of insoluble solids, selenocyanides, thiocyanates and other materials deleterious in the present copper plating baths.

ELECTROPLATING WITH CUPROUS SELENIDE Substantially conventional aqueous cyanide electroplating baths are converted for operation at high current densities merely by the addition of selenides, particularly inorganic selenides. Since inorganic selenides are not particularly effective as copper brighteners, the secondary brightener referred to above is required. The secondary brightener, according to this invention, is the bath-stable organic compound methylene-bis(naphthalene sulfonic acid), sometimes referred to in this specification as MBS."

The quantity of cuprous selenide employed should be enough to supply between about 2 and 50 p. p. m. of selenium to the copper cyanide bath. The upper limit is determined by the loss of brightness range as the concentration of selenide increases while the lower is set by the loss of brightening action below a concentration of about 2 p. p. m. of selenium. The optimum brightening action is found with a concentration of about 2-15 p. p. m. of selenium supplied as cuprous selenide. The selenide ion apparently remains as such in the bath and can be precipitated by lead. The selenide is, of course, preferably added to the bath in solution as pointed out above. Much more selenide than 50 p. p. m. can be dissolved in the bath if a restriction of the bright range can be tolerated.

MBS is the condensation product of aor fl-naphthalene sulfonic acid and formaldehyde, the mole ratio of the constituents varying between 1:2 and 2:1. The material is readily available commercially as the polymer containing about 6-8 naphthalene groups (acidzformaldehyde mole ratio of 1:0.9). Polymers of lower or higher molecular weight are acceptable, bath solubility being the limiting criterion, but that containing about 6-8 naph thalene groups is preferred. aand fl-products are both usable. I

MBS itself is like cuprous selenide, a mediocre brightener for copper. Alone, neither of the agents can produce commercially acceptable bright plate. The reciprocal enhancement of brightening obtained from the two agents is therefore synergistic.

The quantity of MBS is not sharply critical, particularly in the upper limit. Between about 200 p. p. m. and 1000 p. p. m. are preferred. Some synergistic brightening action in combination with the minimal amount of selenide, i. e., around 2 p. p. m., occurs with as low as about p. p. m. of MBS. Up to 2000 p. p. m or even more can also be used but without any improvement over the 1000 p. p. m.

The alkali metal salts of the methylene-bis(naphthalene sulfonic acid) can be substituted for the acid itself. In fact, these salts will generally be formed under the alkaline conditions of the cyanide plating baths. The quantities given refer to the acid added to the bath and to weights in the bath in terms of acid. If salts are utilized or determined instead of the acid, corresponding adjustments in the weights should be made.

There are numerous substantially conventional aqueous copper cyanide baths to which the synergistic cupric selenide-MBS brightener may be added. Preferably the lustrate, but not to limit, the invention.

baths are made up from potassium salts and contain, in aqueous solution:

The free cyanide is that measured by the Liebig silver nitrate titration.

Sodium compounds may be substituted in whole or in part for those of potassium. Since the sodium compounds are cheaper, the substitution is frequently made. The potassium formulations, which permit higher rates of copper deposition and produce slightly brighter deposits, are technically preferred.

Conditions for the operation of-the bath are substantially those of conventional direct-current cyanide plating. Thus the temperature may be varied between about 60 and 95 C. with about 80 C. preferred. Agitation is used to improve the plating speed.

Because of the effect of the selenide ion in repressing hydrogen formation, discussed in some detail in the-parent application, a very wide bright plating range may be employed. This may normally extend from about 3 to about 140 a./s. f. Still higher current densities can be successfully utilized.

The plating current can be reversed periodically if it is so desired. In a preferred cycle the ferrous or other base to be electroplated is made cathodic for about 60 seconds and then anodic for about seconds. The current can also be periodically interrupted. A sequence in which the base is made cathodic for 10 seconds and the current is then stopped for about 1 second is satisfactory. Both current reversal and current interruption tend to prevent polarization in the cell and produce better deposit distribution.

The bath concentrations of selenide and MBS must be maintained at about 2-50 p. p. m. of Se supplied as Cu Se and at least 100-1000 p. p. m. of MBS respectively. Periodic addition of cuprous selenide in aqueous cyanide solution is convenient and easily carried out. As pointed out above, both the entire selenide and cyanide requirements of the bath can be supplied by the same addition, if it is so desired. MBS is also preferably added in aqueous solution but separately from th selenide.

The discussion has so far been restricted to cuprous selenide, Cu Se, alone. Cupric selenide, CuSe, can. also be added to the baths. The copper in the higher valent form is, however, less preferred since it is reduced in cyanide solution to the cuprous form and dissolves only EXAMPLES There follow some examples which are intended to il- The quantities of selenide utilized are reported in terms of cuprous selenide rather than of selenium. Percentages given are weight percentages.

Example 1 This example shows electroplating tests carried out in a Hull cell in accordance with the invention.

A basic aqueous solution, bath K, was prepared conraining (1) l2 oz./gal. of copper cyanide, dissolved in :15

sufiicient potassium cyanide to yield 1.3 oz./gal. of free KCN, and (2) 2.3 oz./gal. of potassium hydroxide. Portions of this solution were employed in two series of plating tests conducted in a standard Hull cell. The Hull cell plates a conductive panel serving as a workpiece with a high. current density at one end and with a low current density at the other. Plating at intermediate points occurs at intermediate current densities. The appearance of the deposit indicates the performance which can be expected on intricately shaped objects plated in large commercial tanks.

The present tests were carried out at C. with a total direct current of 4 amperes maintained for 3 minutes. The anodes were sheet copper and the cathodes brass panels. Brass was chosen as the base metal to facilitate observation of the results but iron or other metallic conductive base can be substituted. The bath was agitated during the tests by a glassv rod passing over the surface of the cathode at 20 ft./min.

A. One series of panels was plated from solutions prepared by adding ingredients to part of Bath K .to determine primarily the brightening action of cuprous selenide when used alone. Such runsmay be considered as controls. Results and important variables are shown in Table I. Two brightness ranges are included; for runs where appropriate. 7 V

TABLE 1 Brightening Range of Current Agent (g./l.) Appearance of Current Density at Run Deposit Densities which H2 (a./s. i.) is Evolved Curse MBS (a./s. f.)

1 None None. Dull to hazy bright. 10-110 2 0.001 do- Hazy-bright; 10-115 115 3 0.002 d0 d 10-120 4 0.005 do. 120 1 5 5 0.01 -.-do 20 a6 1 1 6 M2 '"{D g 145 7 0.05 do 1030 8 0.05 2 Extremelybrightl 20-140 145 B. A second series of panels was plated to determine the synergistic brightening effect of cuprous selenide and MBS (also shown in run 8 above). A fresh sample of bath K was again utilized as the basic solution. Results are shown in Table II.

TABLE II Brightening Range of Current Agent (g./l.) Appearance of Current Density at Run Deposit Densities which H2 (a./s. f) is Evolved OuzSe MBS (a./s. f.)

O Dull t0 hazy-brighL 10110 115 1. 0 Uniform hazy-bright. 10-110 115 1. 0 Extremely bright. 10-115 115 1.0 do 10-120 120 1.0 10-126 126 1.0 10-135 1. 0 10-135 1.0 10-140 Example 2 in total area, the solution being agitated uniformly by an electric stirrer. Under these conditions, panels plated for 30 minutes at 10 amperes total current were dull.

The edges were rough and appeared burned. Profuse hydrogen evolution occurred at currents greater than amperes.

When 0.23 g. of cuprous selenide and 2.3 g. of MBS were added to the bath, the current could be increased to 16 amperes before hydrogen was evolved. Steel panels plated for 1 hour at -16 amperes, total current, were smooth and exceptionally bright. Thus an addition of 0.020 g./l. of Cu se in combination with 0.20 g./l. of

MBS permits a 50-60% increase in plating speed while producing exceptionally bright and smooth deposits.

Example 3 This example shows the maintenance of a fairly large plating tank by the-use of Cu Se-KCN mixtures.

A gallon plating tank containing inorganic selenide was operated for 10 weeks of intermittent, substantially conventional, bright plating. The selenide concentration was readily maintained by periodic additions of a mixture comprising 5% Cu- Se and 95% KCN. The mixture was dissolved in water before addition to form a solution of approximately 25% concentration.

The composition of the bath was as follows:

Component: Quantity CuCN 1 1-12 oz./ga1. KCN (free) 1.2-2.2 oz./gal. KOH 1.5-3 oz./gal. K CO 2-10 oz./gal. MBS 500-1000 p. p. m. Cu se 10-30 p. p. m.

Example 4 This example shows the results of tests on the watersolubility of Cu Se-KCN mixtures.

A. A series of 1 g. mixtures of Cu Se and KCN was prepared and each mixture shaken with 10 ml. of H 0. The Cu se was in the form of a powder passing through a 200 mesh screen but retained on a 325 mesh screen, screen size being reported in the terminology of the National Bureau of Standards. The very soluble KCN was also in the form of a powder. Results are given in The samples are designated as completely soluble if they dissolve entirely when shaken for about 1 0r 2 minutes. No substantial difference in solubility was noted between the tests run at 22 C. and those run at 80 C.

B. 20 ml. of water was added to sample 6 and the temperature of the diluted material was raised to 80 C. The precipitate still did not dissolve.

C. All samples containing a precipitate were allowed to stand at room temperature, i. e. about 20-22 C., over a weekend. No increase in the quantity of selenide dissolved occurred with any sample.

D. A 1 g. sample of 5% Cu Se95% KCN was added to a conventional 12 gallon cyanide bath at 80 C. Dissolution was ahnost instantaneous, no solid reaching the bottom of the tank. A sample consisting of l g. of pure Cu Se failed to dissolve in the bath within a period of several days.

' ADVANTAGES This invention possesses several advantages which will be evident from the preceding discussion. One advantage which is perhaps not so readily evident is the extraordinary stability of baths containing cuprous selenide and MBS. These baths can stand idle for several weeks without forming a precipitate and then produce plate as bright as that originally produced.

An additional advantage of the invention is that the preferred additives, i. e. solid mixtures of metallic cyanides and cuprous selenide, are also very stable and can be kept in the dry state indefinitely. Concentrated solutions, on the contrary, cannot be stored for any length of time.

Another advantage is the fact that only one variable, the concentration of Cu Se, needs careful control. MBS is so soluble that a large amount can be dumped in initially and the baths run for several weeks without further additions.

A further advantage is that where the preferred concentration ranges of the solids are utilized, all the cyanide necessary to run the bath can be added with the cuprous selenide. This procedure also simplifies control of the baths.

A still further advantage is that other adjuvants are unnecessary in the selenide-MBS baths. MBS is an anti-pitting agent as well as a brightener. Unlike many anti-pitting agents it is non-foaming and permits the use of relatively violent air agitation in the baths. Other brighteners are, of course, not required although they are generally harmless and need not be rigidly excluded. Large amounts of the selenocyanide and thiocyanate ions are, however, clearly undesirable because of their ill effects at higher concentrations.

Having described my invention, I claim:

1. The method of producing bright copper plate which comprises electrodepositing the same at a current density of about 3-140 a./s. f. from an aqueous bath held at a temperature of 60-95 C., said bath containing 1-10 oz./ gal. of potassium hydroxide, 0.5-2.3 02/ gal. of free potassium cyanide, 7-20 oz./gal. of copper cyanide, 2-50 p. p. m. of selenium supplied as copper selenide and at least p. p. m. of a member of the group consisting of methylene-bis (naphthalene sulfonic acid) and the alkali metal salts thereof.

2. The invention of claim 1 in which at least part of the potassium ion is replaced by the sodium ion.

3. In the electrodeposition of bright copper from an aqueous bath of copper cyanide, the step of maintaining in said bath (1) between 2 and 50 p. p. m. of selenium supplied thereto as copper selenide and (2) at least 100 p. p. m. of a member of the group consisting of methylene-bis(naphthalene sulfonic acid) and the alkali metal salts thereof.

4. The invention of claim 3 in which the concentration of selenium is 2-15 p. p. m.

5. The invention of claim 3 in which the concentration of said member is 200-1000 p. p. m.

6. The invention of claim 3 in which the copper selenide is cuprous selenide.

7. The invention of claim 6 in which the cuprous selenide is supplied to the bath in aqueous solution with an alkali metal cyanide.

8. The invention of claim 7 in which the concentration of cuprous ion in said aqueous solution does not exceed that required by the formula M Cu (CN)- where M is an ion of the group consisting of potassium and sodium ions.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF PRODUCING BRIGHT COPPER PLATE WHICH COMPRISES ELECTRODEPOSITING THE SAME AT A CURRENT DENSITY OF ABOUT 3-140 A./S. F. FROM AN AQUEOUS BATH HELD AT A TEMPERATURE OF 60-95*C., SAID BATH CONTAINING 1-10 OZ./GAL. OF POTASSIUM HYDROXIDE, 0.5-2.3 OZ./GAL. OF FREE POTASSIUM CYANIDE, 7-20 OZ./GAL. OF COPPER CYANIDE, 2-50 P. P. M. OF SELENIUM SUPPLIED AS COPPER SELENIDE AND AT LEAST 100 P. P. M. OF A MEMBER OF THE GROUP CONSISTING OF METHYLENE-BIS (NAPHTHALENE SULFONIC ACID) AND THE ALKALI METAL SALTS THEREOF. 